Evaporating fuel, which leaks into the air from a fuel tank, a carburetor float chamber, and the like, contains a large quantity of hydrocarbons (HC). The evaporating fuel is described as one of causes of air pollution, and is also responsible for fuel loss. Accordingly, various techniques for preventing such occurrence are known, and there is an evaporation system representative of one such technique. In the evaporation system, evaporating fuel from the fuel tank is at first absorbed and retained in a canister that contains an absorbent such as activated carbon. Thereafter, the absorbed and retained fuel is released (purged) from the canister during operation of an internal combustion engine, thereby supplying the fuel to the engine.
As a fault-diagnosing device for the above evaporation system, one example is disclosed in Japanese Patent Application Laid-Open No. 4-362264. According to the fault-diagnosing device disclosed in this publication, a diagnosis control valve is closed while a purging control valve is opened immediately after engine start-up and at an engine temperature not exceeding a predetermined value. A negative pressure is thereby introduced from an air intake tube into a vapor passageway. Then, the purging control valve is opened and retained in that position for some period of time. In this way, failures are detected on the basis of variations in a pressure level within the above definite period of time. As a result, a small amount of vapor leakage as well as a large amount of vapor leakage from the entire evaporation system including the vapor passageway is detected without possible misdiagnosis.
Now, a course of action for diagnosing failures, which is provided by conventional fault-diagnosing devices for evaporation systems, will be described with reference to a time chart of FIG. 6.
Basically, an air open-close solenoid valve for the canister, which functions as a valve for receiving and blocking ambient air, is initially closed to establish a negative pressure within a fuel tank via purging duty. Values of the purging duty are defined by a traveling state.
When a predetermined level of negative pressure is achieved within the fuel tank, more specifically, when the internal pressure of the fuel tank is equal to a preselected pressure level, the purging duty is set to zero so as to detect subsequent pressure variations after a predetermined period of time elapses. Thus, any leakage from the evaporation system is diagnosed.
That is, assume that the internal pressure of the fuel tank is PT1 when the air open-close solenoid valve for the canister provides a closing action. Further, assume that the internal pressure of the fuel tank is PT2 when the purging duty fluctuates. Then, situations such as evaporation differences due to gasoline properties or capacities are assessed on the basis of the result of PT2 minus PT1.
In addition, assume that the internal pressure of the fuel tank is PT3 when the purging duty is taken as zero. Then, it is ascertained how a purging valve and the air open-close solenoid valve behave while the internal pressure varies from PT2 to PT3.
Moreover, assume that the internal pressure of the fuel tank is PT4 when the air open-close solenoid valve for the canister provides an opening action in response to detection of pressure variations after the predetermined period of time elapses from the moment the internal pressure of the tank achieves a preselected level of pressure. Then, the result of PT4 minus PT3 determines leakage from the tank.
At the final stage, it is determined whether or not the equation (PT4-PT3)-(PT2-PT1) exceeds a predetermined value. When the result is positive, a determination is made that there is a leak.
However, the purging duty for establishing the negative pressure within the fuel tank employs a purging efficiency map which is identical to that used for a normal traveling period. As a result, there is an inconvenience in that a purging efficiency indicative of the purging duty varies with the traveling state.
Consequently, another inconvenience arises in which a high temperature of outside air and a large amount of gasoline evaporation within the fuel tank preclude the pressure within the fuel tank from dropping to a predetermined level of negative pressure. (See light lines of FIG. 7.)
Conversely, when leakage is determined at a low temperature of outside air and a high purging efficiency, there is an inconvenience in that the predetermined level of negative pressure within the fuel tank is reached at such a rapid velocity that an undershooting of the pressure is created so as to preclude stable pressure measurement. (See broken lines of FIG. 7.) This is disadvantageous in practical use.
To obviate the above-described inconveniences, one aspect of the present invention provides a fault-diagnosing device for an evaporation system, in which a canister for absorbingly retaining evaporating fuel is disposed midway along a passageway intercommunicating an air intake passageway of an internal combustion engine and a fuel tank, an air open-close valve being provided for the canister, the fault-diagnosing device further having a control means for effecting control such that the air open-close valve is caused to close when determination conditions are fulfilled, thereby establishing a negative pressure within the fuel tank on the basis of purging duty, and that the purging duty is set to be zero when a predetermined level of pressure is achieved within the fuel tank, thereby detecting a leaking state on the basis of pressure variations that occur after purging is ceased, the fault-diagnosing device being characterized in that the control means is provided with an additional function for setting the purging duty to a predetermined fixed value when the determination conditions are met, the predetermined fixed value being varied in accordance with temperatures and residual fuel amounts.
Another aspect of the present invention provides a fault-diagnosing device for an evaporation system, in which a canister for absorbingly holding evaporating fuel is placed midway along a passageway intercommunicating an air intake passageway of an internal combustion engine and a fuel tank, an air open-close valve being provided for the canister, the fault-diagnosing device further having a control means for effecting control such that the air open-close valve is caused to close when determination conditions are fulfilled, thereby establishing a negative pressure within the fuel tank on the basis of purging duty, and that the purging duty is set to be zero when a predetermined level of pressure is achieved within the fuel tank, thereby detecting a leaking state on the basis of pressure variations that occur after purging is ceased, the fault-diagnosing device being characterized in that the control means is provided with an additional function for monitoring a pressure gradient within the fuel tank when the negative pressure is established within the fuel tank, whereby feedback control over the purging duty is effected in accordance with the monitored pressure gradient.
According to the present invention having the aforesaid structure, when determination conditions are fulfilled, purging duty is set to a predetermined fixed value which is varied in accordance with temperatures and residual fuel amounts. As a result, variations in the purge duty are eliminated, which provides improved precision for determination.
In addition, when a negative pressure is established within the fuel tank, a gradient of the pressure therein is monitored to provide feedback control of the purging duty in accordance with the monitored pressure gradient. As a result, variations in the purging duty are eliminated, which provides enhanced precision for determination.